30-YEAR ATMOSPHERIC CORROSION PERFORMANCE OF 55-PERCENT ALUMINUM-ZINCALLOY-COATED SHEET STEEL

In 1964, a series of aluminum-zinc (AI-Zn) alloy coatings mere applied to steel sheet on a laboratory continuous hot-dip coating pilot line. The coated sheets were exposed in outdoor corrosion tests in severe m arine, moderate marine, rural, and industrial atmospheres. Following e ight years of testing, the 55% Al-Zn composition was selected as the o ptimum composition because it combined excellent long-term durability with the ability to provide cut-edge protection to the steel substrate . Now, after 30 years of continued outdoor testing, the results show c onclusively that the 55% AI-Zn alloy coating has better that twice the life of an ordinary zinc coating of equal thickness, and that it prov ides enduring cut-edge protection. Following identification of the opt imum composition in 1972, steel sheet with the 55% AI-Zn alloy coating was produced commercially by Bethlehem Steel. Large quantities of thi s material have been put in service as unpainted roofing on metal buil dings. Inspections of these buildings show that the corrosion performa nce is excellent for roofs that have been in service for up to 22 year s in a variety of US environments. These results confirm the conclusio ns of the earlier outdoor tests.